In an image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes an optical beam scanning apparatus that forms an electrostatic latent image on a photoconductive drum by irradiating a laser beam (a luminous flux) on the surface of a photoconductive drum and scanning the laser beam.
Recently, besides a monochrome machine including a scanning optical system that uses a single light source, a tandem color machine is proposed. For the tandem color machine, a method of increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) in one laser unit (a multi-beam method) is proposed for the purpose of realizing an increase in speed of scanning on the surface of a photoconductive drum. In the multi-beam method, plural beams for each of color components (e.g., yellow, magenta, cyan, and black) emitted from the respective light sources are subjected to processing in a pre-deflection optical system and are changed to one beam and made incident on a polygon mirror. The beam deflected by the polygon mirror is, after passing through an fθ lens configuring a post-deflection optical system, separated into beams for each of the color components and irradiated on a photoconductive drum for each of the color components.
There is also proposed a color image forming apparatus including an optical beam scanning apparatus that forms plural scanning lines by separating one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, imaging the luminous fluxes by a post-deflection optical system. For example, according to JP-A-7-256926, there is known a technique for making plural beams incident on an identical surface of one polygon mirror, separating the beams in order of height in a sub-scanning direction after passing through one set of fθ lenses, and reflecting the separated beams.
Moreover, recently, there is also proposed an optical beam scanning apparatus in which an individual imaging lens is provided for each of color components in a post-deflection optical system in order to improve optical accuracy in a scanning optical system in the optical beam scanning apparatus. For example, according to JP-A-2003-5113, there is known a technique for making plural beams incident on an identical surface of one polygon mirror, separating the beams in order of height in a sub-scanning direction after passing through a shared fθ lens, making the separated beams incident on an individual fθ lens after reflecting the beams.
However, in the optical beam scanning apparatus in the past in which the individual imaging lens is provided for each of the color components in the post-deflection optical system, the beams separated in order of height in the sub-scanning direction after passing through the shared fθ lens are reflected in order from a beam on a downstream side in the sub-scanning direction with respect to respective photoconductive drums arranged at predetermined intervals in the sub-scanning direction. Reflection mirrors that fold plural beams emitted from the shared fθ lens are arranged further on an upstream side of optical paths for luminous fluxes further on the downstream side in the sub-scanning direction Therefore, it is difficult to reduce a size of the optical beam scanning apparatus.